mckinnis



Jan. 3l, 1956 PREPARATION A. C. MCKINNIS Filed June 16, 1952 OF ISOPHTHALIC AND TEREPHTHALIC ACIDS rra/Flix United States Patent PREPARATION F ISOPHTHALIC ITEREPHTHALIC ACIDS Art: C..McKinnis, Long Beach, Calif., assignor` to Union Gil Companyof California; LosfAng'eles, Calif., a corporation ot'` California Application June 16, 1952, stremo; 293,853A 7 claims. (en 26o- 525)Y Thisiinvention relatesto the'A separation and purication of-` mixtures: of isophthalic1and terephthalic acids, which mixtures-may contain inaddition certain monocarboxylic acids' such asf benzoic or toluic acids. Acid mixtures of thischaracter may be obtained as a result of various oxidation procedures in which mixtures of the isomeric xylenes, with or without ethylbenzene, are oxidized, either in a single stage orin a plurality of stages to formmiie tures of theaboveenamed acids. The particular methods employed-hereinfor separating the acid's'are basedfupon the discovery thatthe esterication rate of' terephthalic acid underthe conditions herein described is substantially lessf than that of isophthalic acid, or of'benzoic or toluic acids.

The process consistsl more specifically in subjecting a mixture-ofthe above acids to^fractional esteriiication with analcohol'. or other esterifying `agent untilsubstantially. all of the monoearboxylic acidsand thel greater' part of; the isophthalic acid have been converted tothe correspondingesters, and thenseparating from thev mixture the substarrtially unesteried terephthalic acid. The invention alsoembraces a partial hydrolysis of the. ester fraction of the-'esterificationmixture in order toselectively de-esterify any terephthalic acidr whichA was esteried, the terephthalic esters being more readily saponiliable than the isophthalic, toluic, or benzoic esters. The remaining mixture of unhydro'lyzedesters isvthen separatedinto its components, as will be more particularly described hereinafter.

The aromatic dicarboxylic. acids, particularly tereph` tlialic`acid; have recently become highly important com-v mercial materialsl by virtue of their use in the manufacture of' certain synthetic polymers such as Baci-on. In thepast, terephthalic acid has generally been manufactured" b'y the controlled. oxidation of vpure para-xylene. Such processes, .as a practical matter, must be performed nitwoiseparatev stages, the iirst stage involving low temperature catalytic oxidation of the para-xylene to paratolnic acid andthe secondstage involving a higher ternperature, liquid phase, non-catalytic oxidation of the toluic acidto terephthalic acid. This process is economicallyV undesirable in that it requires as the starting-material substantiallyV pure para-xylene.

Parafxylene is ordinarily obtained as a by-product from certain petroleum fractions, especially reformate fractions obtained by the hydroforming of naphthenic petroleum fractionsin the presence of certaincatalysts such as cobalt molybdate or platinum. From these aromatic fractions ,mayv be obtained, as. a close boiling, cut, amixture of meta.-, para,` and ortho-xylene and ethyl-V benzene; whichmixture boils between about 135 C..to 1`215""C.'l The` ortho-xylene is the highest boilingmaterial,

boiling"at` 144" Cl'. and is hence ordinarily removed by However, the remaining .mixtureV fractional distillation. offmeta#T andfpara-xylene Y and. ethylbenzene. isV diicult to separate into thetpure components.V A great Vvariety of methods have been .proposedfor separating thismixture,

et' 2,733,266 ce Y Patented Jan- 31, 1956 2, but. all the lcnownV methods are difficult and? expensive. At' present thelmost practical commercial method for sep'- arationy consists ofV repeated? fractional recrystallizations'. However, this method requires the' use of very low tem-'- Vperatures'and a largenumber of crystallization stages in' order to obtain pure para-xylen'e. OtherV methods which have been proposed, such as selective sulfonation, lhave' never been connn'ercially'adopted.' Y

This invention obviates the diiiiculties'involved inthe xylene separation problem by-A providing an economical method'forseparating the oxidation productsoif thev paraxylene,metaxylene', ethylbenzene mixtures: The oxidation of meta-xylene produces either metat-toluic acid or isophthalic acid, depending uponwhether the oxidation is' carried to completion. Para-xylene similarly yields either para-toluic acid or terephthalic' acid, and ortho-xylene gives ortho-toluic acid or plithalc acid.'V Ethylbenzene yields primarily benzoic acidi The present invention makes feasible the oxidation of para-xylene', meta-xylene' mixtures, with or without ethylbenzene, to produce mixe tures of the above acids, and the acids areftlien separated by selective esterication and saponiiication.l

it is therefore the principal objectV of this-inventionto provide economical means for separatingA mixtures comprising isophthalic and tereplithalic acids.

A further object isto avoid the necessity of separating the-Cs aromatic hydrocarbon-isomers prior to the'oxidation-thereof for the' production of dibasic acids.

A further object is to provide' means whereby` substantially pure terephthalic acid? may' be obtained" fromA mixtures'thereofwith isophtha'lic acid. v l

A- further' Objectis to' provide' economicall meansY for separating oxidation mixturesobtained by the oxidation" of' meta-xylene, p'ara-xylene, ethylbenzene mixtures intoV the respective pure carboxylic acids.

These and' other objects are achieved bythe processi more specifically described'hereinafter.

As indicated'V above, an essential feature" of' thisinventionresidesin the utilization of the'differencein esterication rates under certain conditions of isophthalic and terephthalic acids. In utilizingthis phenomenal a mixture of the: acids is subjected, either inV a continuous or batchwise operation, to esterificationwith, preferably, a lower aliphatic alcoholintlie presence'of an esterication catalyst such as sulfuric' orthydrochloric' acid. The esterilicatiorr reaction" is'carried to any degree of` completionY shortlof complete esterifi'cation' ofthe mixture. At any point short, of" complete esti'eriii'cation,l the unesteriedf fraction' will be found to contain a substantially larger proportionk of' tereph'tha'lic acid than was contained iii` the original mixture; If theesteritcation-is carried: on fora substantial perio'dof time, or underl severe esterication conditions, part' ofthetereplithalic acitlmayl bel come; esterified.: The esterilied terephthalic acid, together! withv the' isophthalic esters,V may thenl be subjected' tol fractional hydrolysis'by heatingin the presence of addedwater" anda hydrolysis` catalyst'such asrsulfuric acid or. hydrochloric acid, preferably with:A continuous removal` of the hydrolyzedalcohol.l Under. these conditions itis found that thefterephthal'ate estersfare hydrolyzed,4 inv preference to thef isophth-alate; benzoic` or toluic'A esters- However, someA ofitheiisophthalic-estersmay be? hydro-H lyzedfalso, and-zit is1hencepreferred`to separatethe total mixture' ofl acids.precipitated-l during hydrolysis andfrelcycle it'tofV the fractional esterication stepffor further" purification;

, The l,unhydroly'zed residue from the 'hydroiysisrreaction consistsof` the isophthalate esters,V together with any benzoicg, or vtoluic estersl wliichf-ntayM ber present. benzoic 'and/ or toluic. esters Lmaythenbe-separated'tfro *Il the isophthalic esters by'. distillation, ori any other-conf venient method. The remaining isophthalic ester may then be utilized as such, or it may be further hydrolyzed to obtain pure isophthalic acid.

While this invention is described particularly with reference to the separation of the mixed oxidation product obtained from the Cs aromatic hydrocarbons, it is equally applicable to the separation of any mixture of isophthalic and terephthalic acids. The esterification agent employed herein is preferably a lower aliphatic alcohol. Methanol is the most satisfactory material, inasmuch as it may be readily removed from the hydrolysis reaction as it is formed, and does not form azeotropes with the water present. This is an important advantage, inasmuch as the alcohol should be substantially anhydrous for recycling to the esterification step. `Other alcohols may be employed such as ethanol, propanol, isopropanol, butanol, tert. butanol, cyclohexanol, etc. In addition, other esterifying agents may be employed than the alcohols, e. g., the corresponding olefins, ethers, or esters.

The process may perhaps be more readily understood by reference to the accompanying flow sheet which shows one feasible method of operation. The invention should not, however, be considered as limited to this particular illustrative procedure.

The mixture of carboxylic acids to be separated is first passed through line 1 into a fractional esterification vessel 2. The esterifying agent, for example, methanol, together with the esterification catalyst such as sulfuric acid is introduced through line 3. The mixture of methanol, acids and catalyst is then heated and agitated in the vessel 2 for a sufficient length of time to give a substantial but incomplete esterification of the mixture. The temperature employed may be the reflux temperature of the mixture, i. e. from about 70 C. to 100 C., or a pressure vessel may be `employed and the temperature raised to 300 C. to 400 C. It is preferable to employ an excess of methanol, such for example, as about l to 15 moles thereof for each mole of carboxyl groups present in the mixture. It is also distinctly preferable to employ in the esterification step only suicient of the acidic catalyst to give a reasonably rapid reaction rate; higher proportions of catalyst tend to increase the solubility of terephthalic acid, and thereby increase its relative esterification rate. In using sulfuric acid it is preferable to employ between about 0.1% and 10% by weight thereof, relative to the total reaction mixture.

In observing the above conditions it will generally be found that a satisfactory degree of esterification will have taken place in between about 15 minutes and 5 hours. It is preferable that the reaction period be not extended appreciably more than about 5 hours, under the conditions described, in order to minimize esterification of terephthalic acid. However, under milder conditions, longer esterfication periods may be preferred.

After the desired degree of esterification has taken place the reaction mixture, containing esters, methanol, catalyst and a small amount of water of reaction in the liquid phase, and terephthalic acid in the solid phase, is removed through line 4 and filtered, preferably at approximately the reaction temperature, on filter 5. The precipitate is taken off through line 6 and transferred to a vessel 7 for washing with water which is introduced through line 8. Purified terephthalic acid is removed through line 9. The filtrate from filter 5 consists essentially of a methanol solution of the esters of any acids esterified in vessel 2, together with the sulfuric acid. This mixture is then transferred though line 10 to fractional hydrolysis vessel 11, to which water is admitted through line 12. Hydrolysis is then conducted with heating and agitation for a time sufficient to hydrolyze a major proportion of the terephthalic esters, but insuicient to hydrolyze appreciable proportions of isophthalic, benzoic or toluic esters. The conditions for hydrolysis may be approximately the same as those employed in the esterification step, i. e. reflux or higher temperatures, and atmospheric or higher pressures. It is preferable to maintain in the hydrolysis step a sufficient amount of methanol to provide at least partial misciblity between the organic phase and the aqueous phase at the reaction temperature. For this purpose it is desirable to employ a molar ratio of water to methanol between about 0.5 and 2 for optimum hydrolysis conditions. The excess methanol formed by the hydrolysis reaction may be continuously removed through line 13 and recycled to esterification vessel 2. It may be desirable to employ a rectifier at some point in line 13 in order to remove water which may distill overhead in small quantities along with the methanol.

After the hydrolysis has proceeded to the desired extent the total reaction mixture is removed through line 14 and filtered, preferably at approximately the temperature of hydrolysis, on filter 15. During the hydrolysis the terephthalic acid formed precipitates out as a solid, together with small quantities of the other acids, and these solid materials are removed as precipitate from filter 15 and recycled through line 16 to esterification vessel 2.

The filtrate from filter 15 consists of the unhydrolyzed esters together with small quantities of methanol, water and sulfuric acid. This mixture may be treated in one of two different methods, depending upon the nature of the original mixed acids introduced through line 1. If the mixed acids contained only isophthalic and terephthalic acids the filtrate from filter 15 will consist essentially of isophthalic esters. In this case valve 17 is closed and valve 18 opened so that the filtrate flows through line 19 into hydrolysis vessel 20. Ordinarily the mixture will contain sufficient water to continue the hydrolysis, but if desired additional water may be admitted through line 21. The hydrolysis is continued in vessel 20 until the isophthalic esters are substantially completely hydrolyzed, under the general conditions described for the fractional hydrolysis in vessel 11. The methanol generated by the hydrolysis is recycled through line 22 to esterification vessel 2. The reaction mixture from vessel 20 consists of a slurry of isophthalic acid together with excess water, methanol and sulfuric acid. This mixture is transferred through line 23 to filter 24. The filtrate from filter 24 consists essentially of aqueous sulfuric acid, which may be dehydrated and recycled to esterification vessel 2. The precipitate from filter 24 is transferred through line 25 to washing vessel 26, to which water is admitted via line 27. The purified isophthalic acid is then removed through line 28.

If the original mixture of organic acids contained also benzoic and/or toluic acid, the filtrate from filter 15 may contain, in addition to the isophthalic esters, the esters of benzoic and/or toluic acds. In this illustrative case, valve 18 is closed and valve 17 opened, whereby the liquid filtrate is cooled in heat exchanger 29 and passed through line 30 onto filter 31. The mixture is preferably cooled to between about 0 C. to 50 C., thereby causing solid precipitation of the total ester content, if the benzoic and/ or toluic ester content is not too high, e. g. over about 15%. If the ester mixture contains too large a proportion of monobasic esters, e. g. over about 15%, difliculty may be encountered in precipitating the esters as a solid phase due to the formation of eutectics. In this case the esters may form a liquid phase which may be separated by decantation from the aqueous phase. If liquid phaes are formed, the filter 31 is simply replaced by a liquid-liquid phase separator such as adecanter.

The precipitated ester phase from filter 31, or the liquid ester phase from a decanter, is then passed via line 32 into fractionating column 33 where the more volatile methyl benzoate or methyl toluates, if present individually, are removed as overhead through line 34. If both methyl benzoate and methyl toluates are present, they may be separated from each other by fractionation in column 33, the methyl benzoate being taken overhead through line 34, and the methyl toluates removed as4 ar side-cut through linel 34a. The bottoms from dist'm ion column 393 consists essentially of isophthalicesters wh` h. are then transferred throughline 35 to hydrolysis vessel 2,0 and hydrolyzed as. previously described. The

filtrate*from filter 31 consists essentially of water and sulfuric` acid, together with some methanol. This mixturelmay be transferred throughline to hydrolysisvessel 20`inorder to provide catalyst and water-for the hydrolysis .reaction- A s an alternative to removing thek monobasic` esters after the fractional hydrolysis step they may, if desired, be removed directly after the esterification step, i. e. from theA filtrate in, line 10. In thismodification, the heat exchanger 29r and filter 31, or a. corresponding decanter, are interposed into line 10, andthe separated ester fraction isy fractionally distilled as above described. The residual dibasic esters are then passed into fractional hydrolysis, vessel 11 and treated as previously described. The eliluent fromiilter 15 isthen. passed directly to hydrolysis .vessel 20 throughline 19.

It* is.. possible to obtain from. the esterication proceduresdescribed herein either half esters or diesters of the, dibasic acids, depending upon` the` reaction conditions and, molar, concentrations of the reactants. However, the greater solubility of the. half esters in the alcoholic reaction` mixture, as compared to the diacids, ordinarily favorsgthe formation of the full diester. In any event, the totally unesterifled dibasic acids make up thepredominant partl of` the insoluble residue-from the esterifcation, and theliquidlester fraction may contain a mixture of halfestersgand diesters, but'` very little diacid. Consequently, when the terms esterify or esterificationare applied hereimwithreference to thedibasic acids, those terms are intended to, meanseither monoesterificationor diesterification,

Similarly, in. the. partial hydrolysis step, either halfestersl or the diacids may be. formed. In this casethe half-esters remain in the liquid phasel with the diesters, whilethediacids precipitate as a solid phase; Hydrolysis isv thereforef intended to mean hydrolysis of the esters -to acids which are largely insoluble in the reaction mixture, i. e. diacids.

Theprocesses as described above. are found to provide both terephthalicandzisophthalicacids in purities ranging Ordinarily such purities are from 90%` to over 99%. sufficient for commercial utilization, but if desired the individual acids maybe. further purified by conventional washingorsubliming procedures. l

The following specific examples illustrate typical results whichmay be obtained in practicing the invention:

Example I Ag. mixture made up of 40 grams terephthalic acid and 60 grams isophthalic acid is esterified atl atmospheric pressures and reux temperature for 30 minutes with 570 grams methanol and about l2 grams concentrated sulfuric acid. The solideunesterifed acids are. thenlltered from thehotzreaction mixture and washed with cold water. precipitateis then found to consist of.about. 37 grams terephthalic acid and 12 gramsisophthalic acid. The filtrate is then topped to remove excess methanol, and transferred to a hydrolysis vessel. About 100 grams of water is.added (providing a watten/methanol mole ratio-of about 2.0) and the mixture is reuxed for several hours, until hydrolysis is complete. The precipitated acid is then filtered from the hydrolysis mixture, and is found tov consist of about 47 grams isophthalic acid and 2.5 grams terephthalic acid.

This example shows that a relatively short esterification period results in an incomplete but highly selective esterication of isophthalic acid, with resultant recovery of almost pure isophthalic acid. By prolonging the esteriiication reaction period, unreacted terephthalic acid may be recovered from the reaction mixture in substantially uur@ form.; while the esteried fraction willcontain a slightly higher proportion of terephthalic esters. t

Example II The procedure of Example I is repeated except that the esterication reaction is continued for a period of about 5 hours. rl`he unesterilied fraction contains about 34 grams terephthalic acid and less than l gram of'isophthalic acid. Complete hydrolysis of the ester fraction yields a mixed acid consisting of about 58 grams isophthalic acid and 6 grams terephthalic acid.

Example III- A mixture made up of 40 grams terephthalic acidV and 60 gramsV isophthalic acid is esteried with 80 grams of methanol and 6 grams concentratedV sulfuric acid at about 230 C. in a suitable pressure vessel. The reaction is continued for about 2 hourswhefreupon the reactionv mixture -is cooled to about C. and filtered; After washing, the precipitate is found to consist of about 36 grams terephthalic acid with only traces of isophthalic acid.'

The filtrate is transferred to a hydrolysis vessel and' about'120 grams of water is added. The mixture is` partially hydrolyzed by heatingV at atmospheric reflux for one hour. The precipitate formed, after filtration and? tively-hydrolyzed by heating for several hours with added' water. The solid'material is filteredv off and'found to consist of isophthalic acid of about 99% purity, and'in almost the theoretical yield.

Example l V A xylene oxidation'mixture made up of'30 grams terephthalic acid, 49 grams isophthalic acid, 9 gramsv paratoluic acid, 6 grams meta-toluieacid, 4 grams-benzoic acid and about l gram ortho-toluic acid is partially esterified with about grams ofI methanol and 8 grams of sulfuric acid by heatingy the mixture at atmospheric reflux for 3 hours. The hotmixture is then filtered; and the filtrate transferred to a hydrolysis vessel. Theprecipitate, after washing with 50/50 water-methanol, is found to consist of about 2'/ grams terephthalic acid with only traces of other organic acids.

The esterification filtrate is hydrolyzed'by heating for about 90 minutes with added water at atmospheric reflux. The precipitate, consisting mostly of terephthalic and isophthalic acidsis removed by filtration; The hydrolysis filtrate is then cooled to about 10C., causing a sharper separation of the two liquid phases. The phases are separated by decantation, and the ester phase istheu' transferred to a still equipped with a fractionatingcolumn.

About 3.5 grams of methylv benzoate isrrecoveredas' a firstk overhead fraction collected at between -210 C; A

second overhead fraction is collected at between 2l8-235 sel and hydrolyzed at about 250 C. with added water andi sulfuric acid. The hydrolysisV mixture is then cooled and filtered,` and the filter cake washed with a cold water-methangl. mixtureobtained of about 99% purity.

In any of the above examples the methanol may be replaced by equivalent amounts of any other type of esterifcation agent herein disclosed, e. g. ethanol, propanol, isopropanol, butanol, tert. butanol, cyclohexanol, ethylene, propylene, diethyl ether, dibutyl ether, ethyl acetate, etc. to obtain similar results. In some cases slight modifications in procedure will be required which will be readily apparent to those skilled in the art.

It is apparent that the procedures described herein provide remarkably effective means for obtaining substantial- About 40 grams of isophthalic` acid arely pure terephthalic acid and isophthalic acid from mixtures thereof, which may or may not also contain benzoic acid and/ or toluic acids such as m, p, or o-toluic acids. The invention should not however, be restricted to the specific details disclosed, since many variations will be apparent to those skilled in the art which do not depart from the scope or spirit of the following claims.

I claim:

1. A process for obtaining terephthalic acid from a mixture comprising terephthalic and isophthalic acids which comprises subjecting said mixture to fractional esterication with a lower aliphatic alcohol in the presence of an inorganic acid esteriiication catalyst, said acid catalyst constituting between about 0.1% and 10% by weight of the total reaction mixture, continuing said esterifcation for a period of time suflicient to esterify substantially all of said isophthalic acid but insufficient to esterify more than a minor proportion of said terephthalic acid, said period of time in any case not exceeding about hours, separating substantially pure terephthalic acid and an ester fraction from the esterification mixture, subjecting said ester fraction to fractional hydrolysis by heating with a mixture of water, said aliphatic alcohol and said acid catalyst, the mole ratio of water to alcohol in said hydrolysis being between about 0.5 and 2, continuing -said hydrolysis for a period of time suflicient to hydrolyze substantially all of of the terephthalic esters contained therein but only a minor proportion of the isophthalic esters, separating said terephthalic and isophthalic acids from the hydrolysis reaction mixture and subjecting them to a second fractional esterification step similar to said first esteriiication step.

2. A process as defined in claim 1 wherein said lower aliphatic alcohol is methanol, and said esterification and hydrolysis steps are each conducted at between about 70 and 100 C.

3. A process for obtaining terephthalic acid from a mixture comprising terephthalic and isophthalic acids which comprises subjecting said mixture to fractional esteriiication with a lower aliphatic alcohol in the presence of an inorganic acid esteriiication catalyst, said acid catalyst constituting between about 0.1% and by weight of the total reaction mixture, continuing said esterification for a period of time sutiicient to esterify substantially all of said isophthalic acid but insufficient to esterify more than a minor proportion of said terephthalic acid, said period of time in any case not exceeding about 5 hours, separating substantially pure terephthalic acid from the esterification mixture, adding sufficient water to the liquid residue from said separation to give a molar ratio of water to alcohol between about 0.5 and 2, heating the resulting aqueous mixture still containing the original acidic catalyst for a period of time suflicient to hydrolyze substantially all of the terephthalic ester contained therein but only a minor proportion of the isophthalic ester, separating said isophthalic and terephthalic acids from the hydrolysis mixture and subjecting them to a second fractional esteriiication similar to said first esterifcation.

4. A process as defined in claim 3 wherein said lower aliphatic alcohol is methanol and said esterification and hydrolysis steps are each conducted at between about 70 and 100 C.

"5. A process for obtaining substantially pure terephthalic acid and isophthalic acid from a mixture there* 8 of with at least one monobasic acid selected from the group consisting of benzoic and toluic acids which comprises subjecting said mixture to fractional esterifcation with a lower aliphatic alcohol in the presence of an inorganic acid esterification catalyst, said acid catalyst constituting between about 0.1% and 10% by weight of the total reaction mixture, said esteriiicaton being continued for a period of time suicient to esterify substantially all of said isophthalic acid and said monobasic acid but only a minor proportion of said terephthalic acid, separating from the reaction mixture substantially pure terephthalic acid and a liquid ester fraction, subjecting said ester fraction to fractional hydrolysis by heating with a mixture of water, said aliphatic alcohol and said acid catalyst, the mole ratio of water to alcohol in said hydrolysis being between about 0.5 and 2, continuing said hydrolysis for a period of time sufficient to hydrolyze substantially all the terephthalic esters contained in said ester fraction to terephthalic acid but only a minor proportion of the isophthalic esters and monobasic esters to isophthalic acid and monobasic acid respectively, separating the said organic acids formed by hydrolysis and subjecting them to a second fractional esterification similar to said first esterification, subjecting the remaining ester fraction from said fractional hydrolysis to fractional distillation to remove said monobasic esters, and subjecting the residue from said distillation to a second hydrolysis to obtain substantially pure isophthalic acid.

6. A process as defined in claim 5 wherein said lower aliphatic alcohol is methanol and wherein said esterification step and said first hydrolysis step are conducted at between about and 100 C.

7. A process for obtaining substantially pure terephthalic acid and isophthalic acid from a mixture thereof with at least one monobasic acid selected from the group consisting of benzoic and toluic acids which comprises subjecting said mixture to esterification with a lower aliphatic alcohol in the presence of a minor but effective proportion of an acidic esterification catalyst for a period of time sufficient to esterify substantially all of said isophthalic acid and said monobasic acid but only a minor proportion of said terephthalic acid, separating from the reaction mixture a solid phase consisting of substantially pure terephthalic acid and a liquid ester fraction, subjecting said ester fraction to fractional distillation to remove monobasic esters, and subjecting the dibasic ester residue from said distillation to a first hydrolysis for a period of time sufficient to hydrolyze substantially all the terephthalic esters contained therein to terephthalic acid, but only a minor proportion of the isophthalic esters and monobasic esters to isophthalic acid and monobasic acid respectively, removing the said organic acids formed by hydrolysis, subjecting the remaining ester fraction from said hydrolysis to a second hydrolysis to obtain substantially pure isophthalic acid.

References Cited in the iilc of this patent UNITED STATES PATENTS 1,848,724 Jaeger Mar. 8, 1932 2,479,066 Gresham Aug. 16, 1949 2,569,440 Agnew et al. Oct. 2, 1951 FOREIGN PATENTS 623,836 Great Britain May 24, 1949 

1. A PROCESS FOR OBTAINING TEREPHTHALIC ACID FROM A MIXTURE COMPRISING TEREPHTHALIC AND ISOPHTHALIC ACIDS WHICH COMPRISES SUBJECTING SAID MIXTURE TO FRACTIONAL ESTERIFICATION WITH A LOWER ALIPHATIC ALCOHOL IN THE PRESENCE OF AN STITUTING BETWEEN ABOUT 0.1% AND 10% BY WEIGHT OF THE TOTAL REACTION MIXTURE, CONTINUING SAID ESTERIFICATION FOR A PERIOD REACTION MIXTURE, CONTINUING SAID ESTERIFICATION FOR SAID ISOPHTHALIC ACID BUT INSUFFICIENT TO ESTERIFY MORE THAN A MINOR PROPORTION OF SAID TEREPHTHALIC ACID, SAID PERIOD OF TIME IN ANY CASE NOT EXCEEDING ABOUT 5 HOURS, SEPARATING SUBSTANTIALLY PURE TEREPHTHALIC ACID AND AN ESTER FRACFRACTION TO FRACTIONAL HYDROLYSIS BY HEATING WITH A MIXTURE FRACTION OF FRACTIONAL HYDROLYSIS BY HEATING WITH A MIXTURE OF WATER, SAID ALIPHATIC ALCOHOL AND SAID ACID CATALYST, THE MOLE RATIO OF WATER TO ALCOHOL IN SAID HYDROLYSIS BEING BETWEEN ABOUT 0.5 AND 2, CONTINUING SAID HYDROLYSIS FOR A PERIOD OF TIME SUFFICIENT TO HYDROLYZE SUBSTANTIALLY ALL TO OF THE TEREPHTHALIC ESTERS CONTAINED THEREIN BUT ONLY A MINOR PROPORTION OF THE ISOPHTHALIC ESTERS, SEPARATING SAID TEREPHTHALIC AND ISOPHTHALIC ACIDS FROM THE HYDROLYSIS REACTION MIXTURE AND SUBJECTING THEM TO A SECOND FRACTIONAL ESTERIFICATION STEP SIMILAR TO SAID FIRST ESTERIFICATION STEP. 